Glaucoma is a disease of the eye which reduces the field of view of the afflicted person slowly over time. Glaucoma is believed to be associated with an elevated pressure in the eye, although the exact cause of the disease is not known at this time. This possible explanation for the cause of glaucoma, and the structure of the eye itself, will be described with reference to FIG. 1, which is a side view of a human eye 50 and shows its primary internal components.
Light enters eye 50 through the cornea 55. The light passes through the interior of eye 50 and is focused by the lens 53 onto the retina 59 at the back of the eye. The iris 58 acts as an aperture for lens 53 and serves to define the pupil 51, which is that portion of lens 53 not covered by iris 58. Lens 53 serves to divide the interior of eye 50 into the posterior chamber 54 and the anterior chamber 56.
In a normal (non-diseased) eye, the pressure within eye 50 is maintained at a moderate level by an equilibrium between the production of aqueous humor 52 within eye 50 and its discharge from eye 50. Aqueous humor 52 is a fluid contained within eye 50 which is produced by the ciliary body 60 located behind iris 58.
Aqueous humor 52 is secreted by ciliary body 60 of eye 50 and flows around lens 53 to pupil 51. The fluid circulates through pupil 51 into anterior chamber 56, flowing around iris 58 into the "angle" 57 of the eye between iris 58 and cornea 55. Aqueous humor 52 is discharged from anterior chamber 56 through absorption by the sinus venous or canal of Schlemm (not shown), eventually being discharged from eye 50. A thin filtering system known as the trabecular meshwork (not shown) is located in "angle" 57. The trabecular meshwork controls the rate at which aqueous humor 52 flows out of anterior chamber 56 and is absorbed by the sinus venous. In a healthy eye, the trabecular meshwork allows aqueous humor 52 to flow out of anterior chamber 56 at a controlled rate, one that matches the rate at which it is produced by ciliary body 60.
As a person ages, the trabecular meshwork may become less able to pass aqueous humor at the desired rate. As a result, the pressure within the eye can become elevated because aqueous humor 52 is produced at a greater rate by ciliary body 60 than the rate at which it flows out of anterior chamber 56 and is discharged from eye 50. Such a condition is believed to cause glaucoma.
One method of treating a person with glaucoma (high intraocular pressure) is to apply eye drops containing a medication which reduces the pressure within the eye. However, when this treatment is not satisfactory, argon laser trabeculoplasty may be used.
Argon laser trabeculoplasty (ALT) is a procedure in which a series of approximately fifty (50) laser burns having a spot size of fifty microns (50 .mu.m) in diameter are placed around a one hundred and eighty (180) degree region of the trabecular meshwork. The meshwork itself is only about one hundred microns (100 .mu.m) wide and is viewed by the surgeon through a mirrored contact lens which enables the surgeon to view the trabecular meshwork and to properly place the laser beam. The laser burns are made in the "angle" of a patient's eye, the region between the cornea and iris. ALT is believed to work by opening up the trabecular meshwork so that the rate at which aqueous humor flows out of the anterior chamber of the eye is increased. This serves to reduce the intraocular pressure and prevent loss of vision.
FIG. 2 shows the components of a typical retinal photocoagulator laser system 10 used to perform a surgical procedure such as ALT on a patient's eye. Such a system is described in U.S. Pat. No. 5,171,242, issued Dec. 15, 1992, which has a common inventor and is assigned to the same assignee as the present application.
Laser system 10 includes a laser means 12 for generating a beam of laser radiation having a wavelength and intensity suitable for the desired procedure. Laser means 12 is comprised of a suitable laser 14 and a laser control means 16. Laser control means 16 receives power from any suitable outside energy source and delivers it in a controlled fashion to laser 14. Lasers means suitable for ophthalmological therapies generally, and laser trabeculoplasty therapies in particular, are Coherent laser consoles which provide both lasers and laser control means and include Coherent's Novus 2000, as well as Coherent's Argon laser console model no. 920 A and Argon-Krypton laser console model no. 920 A/K.
Laser means 12 is coupled to a laser focusing means 20 by laser transmission means 18. Laser transmission means 18 is generally an optical fiber cable, although any suitable wave guide capable of efficient transmission of laser radiation at desired wavelengths can be used.
Laser focusing means 20 controls the spot size and focus mode of the laser radiation generated by laser means 12. Laser focusing means 20 may be comprised of any system of lenses, mirrors or other construction capable of focusing laser radiation. It is preferred to construct laser focusing means 20 in the form of a lens system with an adjustable focal length to permit the spot size of the laser radiation at the target to be varied.
Optical means 24 for receiving and delivering laser radiation to the eye is coupled to laser focusing means 20. In laser system 10 of FIG. 2, optical means 24 is provided with a first mirror 26 for receiving laser radiation from laser focusing means 20 and delivering it to the eye. Optical means 24 is also provided with a source of visible light 28. Light generated by source 28 is received by second and third mirrors 30 and 32 which reflect the visible light to the eye, thereby providing the illumination necessary for the physician to view the interior of the eye and position the laser radiation.
Magnification means 40 is coupled to optical means 24 opposite the eye to permit the physician to view the trabecular meshwork in greater detail in order to properly position the laser radiation during therapeutic treatment. A suitable magnification means would be a microscope having magnifying capacity suitable for enlargement of the trabecular meshwork. The combination of magnifying means 40 and optical means 24 is frequently referred to collectively as a "slit lamp" by those skilled in the art.
In addition to the apparatus just described, it is necessary for the physician to employ a form of a contact lens 41 to make it possible for the physician to focus an image of the trabecular meshwork. Normally the structure of the eye, in particular the action of cornea 55 of eye 50 shown in FIG. 1, interferes with the physician's ability to see an image of the meshwork. Because the index of refraction of the inside of the eye is greater than that of air, the steeply angled light rays coming from the "angle" of the eye between the cornea and iris are subject to total internal reflection at the cornea/air interface and cannot escape from the eye. A contact lens is therefore used to provide an index matching (or higher index) material at the interface in order to allow the light rays to escape the eye. The contact lens is mirrored to permit the laser beam and light from a visible light source to enter the eye at the steep angles necessary to illuminate the "angle" of the eye.
Contact lens 41 is placed between optical means 24 and the eye. Contact lens 41 is positioned so that it is in contact with cornea 55 through a gel which is placed on the contact lens by the physician. Examples of typical contact lens systems used in conjunction with retinal photocoagulator laser systems for performing ALT procedures are the CGA lens manufactured by Lasag, the Ritch trabeculoplasty laser lens, single and two mirror Gonio laser lenses and Trokel F/3 Gonio laser lens manufactured by Ocular Instruments, Inc., and the RGO Gonio lens manufactured by Rodenstock.
FIG. 3 shows a typical contact lens 41 of the type used in an ALT procedure and its placement over a patients' eye 50 in order to focus visible and laser light into the "angle" of the eye.
Contact lens 41 has a main body 43 having a front face 44 which is placed over eye 50 during the procedure and a flat surface 46 which permits visible and laser light to enter lens 41. Front face 44 has a transmissive portion which allows light to exit lens 41. The outer curvature of front face 44 is similar to that of cornea 55, so that lens 41 fits snugly over a patient's eye. Lens 41 may be fabricated so as to have one or more magnification lenses 48 external to lens body 43, which are placed on various regions of surface 46. Magnification lenses 48 allow the surgeon performing the ALT procedure to obtain an enlarged view of the inside of eye 50. The interior 45 of lens 41 is solid and is substantially transparent to the visible and laser light used in the procedure. Lens 41 has one or more mirrored surfaces 42 positioned in its interior on the inner wall of main body 43. Mirrored surfaces 42 serve to direct visible and laser light which enters lens 41 through surface 46 to the desired region of eye 50.
As mentioned, laser and visible light enters contact lens 41 by means of surface 46. The light may enter lens 41 and be viewed by the surgeon through magnification lens 48, if needed, to improve the image. The light propagates through the substantially transparent interior 45 of lens 41 and reflects off of mirrored surface 42. As indicated in FIG. 3, reflected light rays 80 are steeply angled in order to exit lens 41 at front face 44 and then enter eye 50 at an appropriate angle to illuminate the region of eye 50 in which the trabecular meshwork is located. As an example, during an ALT procedure, light rays 80 typically enter eye 50 at an angle in the range of 50 to 80 degrees to a normal, N, to the surface of cornea 55 of eye 50.
When a light beam passes from one medium to another across a curved surface it is caused to come to a focus because of the difference in the refractive indices of the media. However, if the visible light or laser beam crosses the surface at a steep angle it becomes astigmatically focused, i.e., the focal points of the light beam are different in the sagittal (horizontal) and tangential planes. This leads to a degradation of the image. Astigmatic focusing is undesirable because it reduces image clarity and increases the spot size of the focused laser beam, and therefore can create problems during procedures such as ALT which require precise placement and size of the laser beam.
In a typical ALT contact lens the visible light and laser beam cross two curved surfaces, one between the body of the contact lens and the cornea, and a second between the cornea and the aqueous humor. The visible light and laser beam must cross the surfaces at a steep angle in order to focus into the narrow "angle" between the cornea and iris of a patient's eye. Since the refractive indices (n) of the lens (typically made of plastic, having n=1.48), cornea (n=1.38), and aqueous humor (n=1.34) differ, astigmatic focusing is a natural result of the geometry of the system.
The problem of astigmatic focusing has been recognized by manufacturers of contact lenses used in ALT procedures. Ocular Instruments, Inc. of Bellevue, Wash., produces a Trokel F/3 Gonio Laser Lens which is designed to correct for cornea induced astigmatism. This lens has an appropriately angled, curved front surface which serves to introduce a compensating astigmatism (one of the opposite sign) in order to cancel the astigmatism introduced by the exit face. A disadvantage of this means of correcting for cornea induced astigmatism is that the degree of compensation depends on the angle at which light enters the lens, so that the effectiveness of the lens in reducing astigmatism can vary.
The contact lens typically used in an ALT procedure is a solid body made of glass or plastic. These materials result in a contact lens having an index of refraction which is higher than the optimal value, but the solid body allows mirrors to be conveniently mounted or fabricated on it. A plastic lens has the added benefit of being lighter weight and less fragile than a glass lens. Since the index of refraction of most glass or plastic lenses (n=1.45 to 1.55) is greater than that of the aqueous humor of the eye, the laser beam becomes astigmatically focused as it passes from the contact lens into the eye. This problem can be partially addressed by tilting the contact lens to produce a compensating astigmatism. However, the direction and degree of tilting is often limited by practical considerations, such as the patient's facial features.
ALT is a procedure where a small laser spot is important. As the trabecular meshwork is only 100 .mu.m across, it is desirable to make the laser burn in it as small and clean as possible. A clean 50 .mu.m spot, which is the smallest typically obtainable, is obtained in a photocoagulator by converging the laser beam at a relatively steep angle. Astigmatic focusing can increase the spot size from 50 .mu.m to 90 .mu.m or more. Even if a reducing section of the contact lens is used, which can be obtained by the addition of a "button" lens, the spot size may still be increased to as much as 75 .mu.m. It is apparent that reducing the degree of astigmatic focusing would provide better viewing of the area to be treated and a smaller laser spot size, and hence a more effective and safer procedure for treating glaucoma.
What is desired is a contact lens for use in viewing the interior of the eye and delivering laser energy during an ALT procedure which causes less astigmatic focusing of the laser beam than contact lenses typically used in such procedures.